1. Field of the Invention
This invention relates to a method and apparatus for minimizing the loss of power in all or part of a separately excited electromechanical energy conversion system having, for example, a chopper controller, battery power supply and electromechanical energy conversion device. An electromechanical energy conversion device having n spatially fixed windings, n.gtoreq.1, for producing a magnetic field, and having m movable windings, m.gtoreq.1, for producing a magnetic field which opposes the magnetic field produced by the n spatially fixed windings, is separately excited if the magnetic field produced by the n spatially fixed windings can be varied, at least in part, independently of the magnetic field produced by the m movable windings. For example, in a separately excited dc motor/generator, the n spatially fixed windings can be any combination of constantly excited field windings, series field windings, and shunt field windings, with at least one separately excited field winding. The m movable windings can be armature windings.
Hereafter, unless otherwise stated, the term "field" will be used to refer to the electrical circuit zf field winding(s). Also, the term "armature" will be used to refer to the electrical circuit of armature winding(s). This terminology is common in the literature.
2. Description of the Prior Art
(a) First, the conversion of electrical energy to mechanical energy in a dc motor will be considered. Heretofore, the speed and torque control of a separately excited motor, in the low speed running mode, was carried out by maintaining the field current constant and varying the armature voltage, and in the high speed running mode, by maintaining the armature voltage constant and varying the field current. This type of two speed-range controller was modified by Ohmae et al. (Reference U.S. Pat. No. 4,037,144) as follows
(I) Assume that the field magnetic flux is developed in proportion to the field current I.sub.F.
(II) Assume that the field magnetic flux is independent of the armature current I.sub.A.
(III) Assume that the field circuit resistance R.sub.F is not a function of field current I.sub.F.
(IIIa) For a chopper controller, assume that R.sub.F is fixed.
(IV) Assume that the armature circuit resistance R.sub.A is not a function of armature current I.sub.A.
(IVa) For a chopper controller, assume that R.sub.A is fixed.
(V) Then the electrical power loss W=I.sub.A.sup.2 .multidot.R.sub.A +I.sub.F.sup.2 .multidot.R.sub.F is minimized when ##EQU1##
(VI) Therefore, modify a two speed-range controller to maintain the relationship of I.sub.F to I.sub.A as given in (V), unless the value of I.sub.F thus calculated exceeds minimum or maximum limits. In the latter case, maintain I.sub.F above the minimum limit or below the maximum limit, respectively.
The maximum limit on I.sub.F is stated by Ohmae et al. to be that level of current which causes the field magnetic circuit to be saturated. For a range of I.sub.F below this limit, assumptions (I) and (II) are invalid. The reason for this is that the incremental change in the field magnetic flux caused by a change in the field current is a function of both the field current magnitude and the armature current magnitude. Assumptions (III), (IIIa), (IV) and (IVa) can also be significantly inaccurate. The present invention can achieve higher efficiency because it is not based on the above assumptions.
The implementation of (VI) by Ohmae et al, still requires a two speed-range controller. The present invention provides a simpler apparatus.
In a paper titled "Minimization of Electrical Losses in a Battery Electric Vehicle" by J. Morton, J. Jones, and C. Watson, presented at Drive Electric 80, Wembley, UK, Oct. 16, 1980, another scheme for minimizing I.sup.2 R losses in a motor system was presented. In this scheme, field flux is assumed to be represented by a second degree polynomial in field current, independent of armature current. Armature chopper duty cycle and field chopper duty cycle are controlled simultaneously as a function of accelerator pedal position and motor speed. (No algorithm for determining the appropriate values of armature chopper duty cycle and field chopper duty cycle for different accelerator pedal positions and motor speeds is given.) Since the armature chopper duty cycle and the field chopper duty cycle are controlled and not the armature current and the field current, this method depends on the relationship between chopper duty cycle and chopper current in order for the system optimization to work. One problem is that this relationship is dependent on the stability of circuit parameters which in fact can vary due to manufacturing tolerances, temperature, and component aging. In particular, the relationship of armature current to armature chopper duty cycle is very sensitive.
The present invention makes no assumptions regarding field flux as a function of field current or armature current. Furthermore, the present invention controls motor currents directly, which is a more efficacious way to minimize power loss in the system than controlling chopper duty cycles. Also, the present invention is not limited to minimizing I.sup.2 R losses; all electrical losses can be minimized, such as magnetic losses, which are modelled as I.sup.2 R losses.
(b) Second, in considering the conversion of mechanical energy to electrical energy, no prior references were found regarding power loss minimization.